1. Field of the Invention
The present invention relates generally to nonreciprocal optical devices, and more particularly to arrays of nonreciprocal devices, such as isolators and circulators, integrated on a common planar optical substrate.
2. Description of the Prior Art
Nonreciprocal optical devices, such as optical isolators and optical circulators, are essential components of optical communication systems. Optical isolators pass light propagating in a forward direction while inhibiting the propagation of light in a backward direction. Optical circulators enable the routing of light from one optical fiber or waveguide to another based upon the direction of light propagation.
Commercially available nonreciprocal optical devices generally take the form of individual (non-integrated) assemblies of bulk optical components. For example, optical isolators typically utilize a GRIN lens attached to an input fiber to collimate the input light. The light is then passed through a series of polarization and Faraday rotation components and subsequently recaptured by a second GRIN lens that recouples the light onto an output fiber. Manufacturing of such isolators involve numerous assembly and manufacturing steps (most of which must be performed manually), resulting in high costs and limitations in production volume. The growth and increasingly price-competitive character of the fiber optic equipment industry has created a demand for low-cost nonrieciprocal devices which may be manufactured in large volumes using automated assembly techniques. A particularly strong demand exists for array architectures, in which plural isolators or other nonreciprocal devices are integrated into a single structure.
U.S. Pat. No. 5,706,371 to Pan (xe2x80x9cOptical Isolator Array Devicexe2x80x9d) presents one example of an isolator array architecture. The Pan device consists of corresponding input and output arrays of optical fibers disposed in V-grooves formed on one surface of a supporting substrate. An isolator subassembly, comprising a strip of Faraday material sandwiched between strips of birefringent crystal material, is fixed within a transverse trench formed in the substrate between the input and output optical fiber arrays. Light leaving the input fibers is collimated (either by GRIN lenses located proximal to the fiber endfaces or by at thermally expanded cores) and directed onto the isolator subassembly. The receiving ends of the output fibers are provided with collimating elements (GRIN lenses or thermally expanded cores) to couple light transmitted from the corresponding input fibers through the isolator subassembly.
The approach described in the aforementioned Pan patent does offer certain advantages over existing single-channel designs, but has several problems associated with its implementation. These problems include a need to utilize non-standard fibers having thick ( greater than 200 xcexcm) claddings to prevent excessive losses resulting from the presence of a sizable evanescent field at the cladding outer surface; processing and induced mechanical fatigue issues associated with thermal expansion of the fiber cores, and; difficulty in automating the placement and alignment of the optical fibers and any separate collimating elements (e.g., GRIN lenses). These and other problems associated with the Pan approach may significantly raise manufacturing costs and compromise device performance. There remains a need in the art for an array-based nonreciprocal device which is well-suited for high-volume manufacture by automated methods, and which may be produced relatively easily and inexpensively.
According to a first embodiment of the invention, an integrated isolator array is provided having a plurality of buried waveguides formed in an optically transparent substrate, such as lithium niobate or a glass. Each waveguide is divided into input and output sections. The input sections of the waveguides are preferably adapted with input tapers designed to adiabatically expand the optical mode from a compact size (typically matched to that of standard optical fiber) to a relatively large size. Conversely, the output sections of the waveguides may be adapted with output tapers to adiabatically reduce the optical mode from the large size emerging from the input tapers to a compact size. An intermediate section of the waveguide, extending between the input and output tapers and bisected by the isolator subassembly, provides a path for light propagating in a collimated form from the input to the output section through the isolator subassembly. Fabrication of the waveguides may be accomplished by ion exchange or other suitable techniques that are known in the art. An isolator subassembly, which may consist essentially of layers of Faraday material interposed between layers of birefringent crystal material, is received within a trench formed in the transparent substrate between the input and output sections of the waveguides such that the isolator subassembly intersects the optical paths of the plurality of waveguides.
In another embodiment, a plurality of four-port circulator structures are formed in an optical substrate. Each circulator structure includes a pair of waveguides having first and second sections, each of the first and second sections terminating in a port. A nonreciprocal optical subassembly is fixed within a trench located between the first and second sections and positioned in the optical paths of the waveguide pairs. The nonreciprocal optical subassembly is configured to rotate the polarization of light traveling from the second sections to the first sections of the waveguides while leaving unchanged the polarization of light traveling from the first sections to the second sections. The circulator structure further includes first and second polarization multiplexers respectively coupling the first and second sections of the waveguides. Light entering a port is split by one of the polarization multiplexers into two beams having orthogonal polarizations. The polarized light beams then pass through the nonreciprocal subassembly and are subsequently combined into a single beam by the other polarization multiplexer. The combined beam then exits the circulator structure by an exit port different from its entry port. The polarization multiplexers and nonreciprocal subassembly collectively function to route a first light signal input to a port of the first section of the first waveguide to be output at a port of the second section of the first waveguide, a second light signal input at a port of the second section of the first waveguide to be output at a port of the first section of the second waveguide, and so forth.
The invention further encompasses a doped fiber amplifier array architecture employing an integrated circulator array of the foregoing description. The architecture includes an array of input fibers each carrying an input signal to be amplified, and an array of output fibers each carrying an amplified output signal. Each input fiber is coupled to a corresponding circulator structure via a first port thereof, and each output fiber is coupled to a corresponding circulator structure via a third port thereof. An array of doped fibers are optically coupled at their first ends to corresponding second ports of the circulator structures and receive pump light through their second ends. An input light signal entering a circulator structure is routed to the second port, where it is then coupled into the first end of the corresponding doped fiber. The light signal is amplified as it travels along the length of the doped fiber. A wavelength selective reflector located at the second end of the doped fiber reflects the partially amplified signal, which then travels along the doped fiber in the opposite (backward direction) and is further amplified. The amplified signal is then coupled back into the second port of the circulator. The second ports may be adapted with wavelength selective reflectors, which are highly reflective at the pump light wavelength and non-reflective at the input signal wavelength, to reflect the pump light back into the doped fiber while allowing the amplified light signal to be re-admitted into the circulator structure. The circulator structure routes the amplified light signal to the third port, where it is coupled into the corresponding output fiber. Utilization of the xe2x80x9cdouble-passxe2x80x9d architecture described above, wherein the input light signal and pump light each travel twice (in opposite directions) through a doped fiber offers significant advantages over existing commercial architectures, including compactness, higher gain, lower noise, and lower cost of manufacture.